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Old 05-28-2011, 07:33 PM   #4811 (permalink)
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I've got a way to get rid of the isolation pads now. It started with an idea from someone that got a power board from me (Michael Black is his name), who figured a way to remove the isolation pad. I modified the idea to reduce the loop inductance.

Each mosfet will be bolted to a 1/2" x 1.5" copper bus bar which will also act as the heat spreader. That heat spreader will be bolted directly to a finned heatsink. A fan will suck air across the heatsink fins. The fins will be inside the controller, since they will be connected to M-.

The diodes will be connected to a 1/2" x 1.5" copper bus bar which also acts as a heat spreader. IT will be connected to its own finned heat sink, and will have a fan sucking air across its fins. The fins will be exposed to the outside air, but the electronics inside won't be, so no dirt on the electronics will be possible. The modified power board is almost done. The plan is to use 12 gigamos mosfets, 230amp 200v, and 12 schottky diodes that are rated for 240amp each.

the thermal resistance should be very very tiny. And the equivalent of a 1" x 1.5" x 10" copper block, but smaller loop than anything I've done so far. The mosfets will be able to be mounted so that only the thick part of the legs are right up against the pcb. I think 140amp per device will be possible.

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Old 05-30-2011, 04:07 AM   #4812 (permalink)
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Quote:
Originally Posted by MPaulHolmes View Post
I've got a way to get rid of the isolation pads now...

Each mosfet will be bolted to a 1/2" x 1.5" copper bus bar which will also act as the heat spreader. That heat spreader will be bolted directly to a finned heatsink. A fan will suck air across the heatsink fins. The fins will be inside the controller, since they will be connected to M-.
Cool! Just an experience of mine...

I tried something similar when I was developing the powerboard for my charger. I had the mosfet and diode on different heatsinks so they could be attached with minimal thermal resistance, but I had awful results. The circuit made an audible noise and a scope of Vds looked really weird. I don't really remember, but either it blew up or I didn't feel comfortable pushing it past just a couple of amps.

I think the problem was that the back of the mosfets are connected to the drain pin. This node of a low side buck converter circuit sees a very high dV/dt during switching, so maybe the electrically attached heatsink added a significant amount of stray capacitance which caused bad things to happen. I ended up spending a few extra bucks for a package with an isolated back. Another option may have been going to a high side switch where the drain pin (and heatsink) is now at the positive rail voltage and the source pin sees the high dV/dt.

Of course, I was switching rectified 240 (so like 350VDC), and at a faster frequency and maybe even with harder gate driving. So, it may not be a problem for the motor controller, but it might be something to look out for when testing starts.
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Old 05-30-2011, 08:21 AM   #4813 (permalink)
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Thanks Joe! I didn't think of that one. Yikes... I'm going to try it first on a small scale. Lee hart always says that you should solder the mosfets against a thin strip of copper, and then put the thin strip of copper against the isolating pad. Maybe that's why?
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Old 05-31-2011, 12:55 AM   #4814 (permalink)
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First drive with my controller!



Still alot of work to do but I can start to see the light at the end of the tunnel.
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Old 05-31-2011, 03:45 AM   #4815 (permalink)
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Quote:
Originally Posted by mrbigh View Post
Here is a preview of my version of the OpenRevolt Paul & Sabrina's motor controller version.
.................................................. ....with some open revolt decals


OOOOOOooooooo, AAAAAAAAAhhhhhhh, that's nice!
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Old 05-31-2011, 04:35 AM   #4816 (permalink)
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That's awful! (:-) Looks like a footstool.....
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Old 05-31-2011, 08:55 AM   #4817 (permalink)
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Hey Camaro! Awesome!!!!! Good work! Isn't the first drive scary, but in a good way!?!
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Old 05-31-2011, 11:44 AM   #4818 (permalink)
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Note to future self:.....

When converting a car to electric - BACK it into the garage.

That way, the first official test drive of the car is in a triumphant FORWARD motion coming out of the garage!!!
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Old 05-31-2011, 02:52 PM   #4819 (permalink)
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Thanks guys,
Yep it's a little scare, I couldn't help but to think the drive train was going to fall apart at any moment.
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Old 05-31-2011, 03:26 PM   #4820 (permalink)
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I know that feeling! My first test drive i reversed into a tree

anyway the LCM is nearly done. Wired up the shunt field control components this evening. Am scared of this thing!!

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